Electron beams are used in a variety of instruments, including electron microscopes, e-beam lithography systems, critical dimension measurement tools, and various other inspection, analysis, and processing tools. In most instruments, information about a sample is acquired by observing results of the interaction of the electron beam with the sample. In such instruments, electrons are emitted by an electron source and formed into a beam, which is focused and directed by an electron optical column. An electron source typically includes an emitter from which electrons are emitted; an extraction electrode, which extracts electrons from the emitter; and a suppressor electrode, which suppresses unwanted emission of electrons away from the emitter tip. An ideal electron source produces electrons that can be focused to a nanometer or sub-nanometer scale spot, with sufficient electron current to provide rapid, consistent data collection or electron beam processing. Such an electron source is typically characterized by having a low energy spread among the emitted electrons, high brightness, and long-term stability. Low energy spread reduces chromatic aberration of the beam in the electron column because chromatic aberration is caused by electrons having different energies being focused to different points.
To be freed from a solid surface, an electron must overcome an energy barrier. The height of this energy barrier is referred to as the “work function” of the material. Electron sources can use different types of emitters, which use different methods to overcome the work function. A “thermionic emitter” is heated by a filament to provide the electrons with sufficient thermal energy to overcome the energy barrier and leave the surface. A “field emitter” relies at least in part on an electric field to pull electrons from the source.
A “cold field emitter” uses an electric field to provide the conditions for electrons to tunnel through the energy barrier, rather than providing the electrons with sufficient thermal energy to pass over the barrier. “Schottky emitter” (“SE”), uses a combination of coating materials that lower the work function, heat to provide thermal energy, and an electric field to free the electrons. SEs typically operate at about 1,800 Kelvins. The Schottky electron source has become the most widely used source in electron optical systems where high brightness and/or small energy spread is required. Another type of emitter, a “thermal field emitter” typically operates at a higher temperature than an SE and operates similar to a cold field emitter but only at high temperatures for increased emission stability.
The high electric fields and/or high temperature during emitter operation results in changes to the emitter shape over time. FIGS. 1A-1C show a variety of emitter shapes or “end forms.” The emitter likely will assume all three of these end-forms during a typical life span of 1-3 years. The emitter end forms are referred to Stage 0 (FIG. 1A), Stage 1 (FIG. 1B) and Stage 2 (FIG. 1C). The different crystal facets shown in FIG. 1 grow or shrink, causing the overall morphology of the endform to the change, which in turn causes the field for a given extraction voltage to change, and hence the emission characteristics change.
Emission characteristics of an electron source can be determined by removing the source from the column and installing it in specifically designed, expensive test equipment. It is not currently practical for an operator to determine characteristics of a source “in situ,” that is, with the source mounted in a focusing column, such as in a scanning electron microscope (“SEM”) or transmission electron microscope (“TEM”). Currently available in-situ techniques entail complex procedures beyond the skill of most instrument operators. For example, one currently available method for inferring source brightness in an SEM entails operating the column in virtual source size limited mode and measuring the spot size. This technique depends on the ability of the system operator to align the column to obtain the best spot size and then to make a proper measurement. Because there is no accurate method to evaluate the state of the SE in-situ, emitters are often removed pre-maturely at significant cost for parts and down time for the instrument.
Current knowledge in the field of Schottky emitter is described in part in the following references:    Liu et al., “Field induced shape and work function modification for the ZrO/W(100) Schottky Cathode,” J. Vac. Sci. Technol. B 28 (6) pp. C6C26-33 (2010).    Bahm et al., “Range of Validity of Field Emission Equations,” J. Vac. Sci. Technol. B 26 (6) pp. 2080-2084 (2008).    M. S. Bronsgeest, “Fundamentals of Schottky Emission,” http://tnw.tudelft.nl/index.php?id=33723&L=1, Delft University of Technology    Bronsgeest et al., “Probe current, probe size, and the practical brightness for probe forming Systems,” J. Vac. Sci. Technol. B 26 (3) pp. 949-955 (2008).    Handbook of Charged Particle Optics, 2nd edition, J. Orloff editor, CRC Press (2008)    Modinos, Field, Thermionic, and Secondary Electron Emission Spectroscopy (Plenum Press, N.Y., 1984).    Bahm, et al, J. Appl. Phys. 110 (2011) 054322